1. Field of the Invention
The present invention relates to a projection-type display apparatus that projects and displays an image on a transmission-type screen.
2. Description of the Related Art
The recent trend is to provide a high-brightness light source in order to realize high brightness in a projection-type display apparatus in which a liquid crystal panel or a reflecting optical modulator (a reflecting liquid crystal display or the like) is employed as an image displaying device.
An extra-high pressure mercury lamp, a metal halide lamp, or the like has been used as a light source for the projection-type display apparatus. However, these types of lamps have a shorter lifetime so that a maintenance work to replace them is required frequently. Furthermore, when these types of lamps are employed, an optical system that takes out red, green, and blue from white light of the lamp becomes necessary, so that the configuration of the apparatus becomes complicated and light use efficiency drops.
To solve these problems, a laser source, such as a semiconductor laser, has been tested as a light source. The laser source has a longer lifetime and does not require a maintenance work so frequently. Moreover, the laser source can be directly modulated in accordance with an image to be displayed so that configuration becomes relatively simple with improved light use efficiency. In addition, a color reproduction range can be widened by employing the laser source.
However, the laser source has a high coherence. Therefore, when the laser source is used as a light source in a projection-type display apparatus, a light diffusing material within a transmission-type screen of the projection-type display apparatus interferes with light and thus the displayed image glares (speckle noise or scintillation). Such interference leads to degradation of the picture quality.
In the projection-type display apparatus, it is preferable to reduce the speckle noise or scintillation. For this purpose, a method for prescribing a relationship between an exit pupil diameter and a projection distance of a projection lens and the number of diffused layers of a transmission-type screen has been disclosed in, for example, Japanese Patent Application Laid-open No. H8-313865. Moreover, a method for internally oscillating at least one layer of diffused layers of a transmission-type screen has been disclosed in, for example, Japanese Patent Application Laid-open No. 2001-100317.
The method disclosed in Japanese Patent Application Laid-open No. H8-313865 prescribes a ratio (d/a) of an exit pupil diameter d to a projection distance a of a projection lens to 0.06 or less. However, when the ratio (d/a) is small, there is a problem that a divergence angle of light incident on the transmission-type screen becomes small whereby speckle noise and scintillation is disadvantageously is increased.
On the other hand, the method disclosed in Japanese Patent Application Laid-open No. 2001-100317 has a problem that a mechanism for vibrating the diffused layer of the transmission-type screen is required. Therefore, the size of the apparatus is increased and the cost is increased. Moreover, there is a problem that a device behavior becomes unstable due to vibration of the diffused layer and thus the reliability of image display is not secured.